Today, there are many prospective applications for a high-density multi-electrode biocompatible implant. One of the most important is for a cochlear implant. The cochlea is a snail shaped organ of the inner ear that translates sound waves into electrical nerve impulses. A cochlear implant, by directly electrically stimulating the cochlea can effect hearing restoration in persons otherwise completely deaf and for whom other methods of hearing restoration would be ineffective. Compared to the cochlea, however, which includes approximately 30,000 receptive nerve endings, currently available cochlear implants are crude devices, capable of stimulating the cochlea with a degree of selectivity far beneath what the cochlea is capable of perceiving. Accordingly, in order to effect a more complete hearing restoration, cochlear implants having a higher density of precisely positioned electrodes are needed.
Because the cochlea has so many more sensing sites than an implant could possibly have electrodes, it is desirable to stimulate the cochlea at points between electrodes. This can be effected by xe2x80x9cbeam forming,xe2x80x9d in which neighboring electrodes are separately controlled to form a beam that has its maximum at a desired cochlear stimulation point. Unfortunately, in order to perform beam steering it is generally desirable to have electrodes that are spaced apart by no more than 100 to 150 um. Achieving this fine spacing of electrodes has proven a challenge to researchers.
The cochlea is not the only site within the body where a high-density implant could be of use, however. The brain, the retina and the heart are just a few other sites within the body where such an implant could be used. Unfortunately, providing such an implant proves to be quite difficult in practice.
Among the challenges encountered in the construction of an implant having a large number ( greater than 30) of closely spaced ( less than 100 um) and precisely positioned electrodes is the problem of decomposition in the body due to attack by the body""s interstitial fluid (ISF). Any seam in an implant will be attacked by ISF and may eventually come apart. Because of this, it is extremely important that biocompatible materials be used throughout an implant. Moreover, the more that an implant can take the form of a seamless, unitary whole the longer an implant can be expected to last within the body. This requirement conflicts with the greater level of complexity desired of implants.
Researchers at the University of Michigan have designed one type of probe that is currently in popular use. This probe is made by micro machining a silicon substrate using photolithographic techniques in order to achieve accurate positioning of closely spaced electrodes. Unfortunately the materials used are somewhat brittle. Accordingly this probe is not well suited for an application that requires flexure, such as a cochlear implant.
Additionally, multilayer printed circuit board (PCB) technology has advanced so that multilayer structures having traces on the order of microns thick are now available. There are a number of reasons, however, why this technology has, in general, not been applied to the biomedical arena. First, many of the materials used in PCB manufacture are not biocompatible, or degrade after implantation. Second, even flex circuits made from polyimide, a flexible dielectric, typically do not have the degree of flexibility necessary to facilitate the construction and placement of a cochlear implant.
Accordingly, there is a long-standing, unresolved need for a biocompatible, long-term implant that can precisely stimulate a sensory bodily organ, such as the cochlea.
In a first separate aspect the present invention is a laminated multi-electrode biocompatible implant, comprising a first layer of flexible, biocompatible dielectric material having a first, exposed surface. A second layer of flexible biocompatible dielectric material, is adhered to the first layer and, in turn, a third layer of flexible biocompatible dielectric material is adhered to the second layer. Additionally, a first conductive trace is interposed between the first layer and the second layer and a second conductive trace is interposed between the second layer and the third layer. Finally, a first inter-laminar conductor, which breaches the first layer, conductively connects the first conductive trace to the exposed surface of the first layer, thereby forming a first electrode and a second inter-laminar conductor, which breaches the first layer and the second layer conductively connects the second conductive trace to the exposed surface of the first layer, thereby forming a second electrode.
In a second separate aspect, the present invention is a method of making a biocompatible laminated multi-electrode implant, comprising providing a first lamina of flexible, biocompatible material and constructing a first set of conductive traces on the first lamina. A second lamina of flexible, biocompatible material is formed on top of the first lamina and the first set of conductive traces in such a manner that the second lamina adheres to the first lamina. Then, a second set of conductive traces is constructed on the second lamina. A third lamina of flexible, biocompatible material is formed on top of the second lamina and the second set of conductive traces in such a manner that the third lamina naturally adheres to the second lamina and has an exposed surface. A portion of the third lamina is removed to create a first opening, extending from the exposed surface to a first one of the second set of traces and a portion of the third lamina and the second lamina is removed to create a second opening, extending from the exposed surface to a first one of the second set of traces. Finally, conductive material is introduced into the first opening and the second opening, the conductive material extending through the entire length of the first opening and the second opening and thereby creating a first electrode and a second electrode on the exposed surface.
In a third separate aspect the present invention is a method of creating a bio-implant, having a preset shape, comprising the steps of creating a flexible structure having a set of conductors and electrodes that are connected to the conductors and thermo-forming the structure into the preset shape.
In a fourth separate aspect the present invention is a method of creating a cochlear implant, including the steps of creating a flat spiral structure having a proximal end, a plurality of electrode sites and a plurality of conductors leading from the electrode sites to the proximal end and expanding the flat, spiral structure into a three dimensional helix.
The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.